Determination of Fluvial Sand Layer Using CPTu Test Results for an Underground Project in Singapore
Lanting WU1, Jianping SUN2#+, Siew Ann TAN3
1China Communications Construction Company Limited (Singapore Branch), Singapore, 2CCCC-SJ PTE LTD, Singapore, 3Department of Civil and Environmental Engineering, National University of Singapore, Singapore

A deep excavation was carried out at eastern part of Singapore which was 50m wide and 20m deep based on the dimension of permanent structure. Steel pipe pile wall were proposed as the retaining wall. Typical soil profile consists of 10m to 16m thick of reclaimed Fill, 7m to 32m thick of Kallang Formation (Fluvial SAND and CLAY and Marine CLAY) and competent Old Alluvium of varying weathering degree. There was potential risk of the ingress of water into excavation if the pipe pile interlock was not grouted when fluvial sand was encountered. Therefore, the fluvial sand layer shall be determined by soil investigation.The cone penetration test (CPTu) allows for a continuous soil profiles and can collect independent readings in a single sounding. These readings, notably the cone tip resistance (qc), sleeve friction (ft) and penetration pore water pressure (u2) are interpreted to give the soil parameters used to assess subsurface stratigraphy. Since both the penetration resistance and sleeve resistance increase with depth due to the increase in effective overburden stress, the CPTu data requires normalization for overburden stress. And normalized soil behavioral type (SBTn) chart proposed by Robertson (1990) is commonly used. Another parameter, Soil Behavior Type Index Ic, is also commonly used to represent the SBT zones in the SBTn chart. Furthermore, Robertson (2010) proposed a simplified relationship between soil permeability and Soil Behavior Type Index, Ic and this relationship is used in this study.A series of cone penetration (CPTu) tests were carried out and Ic was obtained based on normalized cone resistance and normalized friction ratio. Then the permeability was estimated using the relationship between soil permeability and Ic, and the fluvial sand layer was determined when the permeability is higher than 10-6 m/s. Finally, the interpretation of CPTu test results was used in the design successfully.